The stimuli used in the evoked potentials are electrical or laser. They are started and synchronized with the collection of the EEG by signals TTL (transistor-transistor logic). Investigators propose to validate a pneumatic stimulator delivering the compressed air sync with the EEG. It has two advantages over existing stimuli: Is capable of inducing in patients an allodynic response, excessive, painful, in response to a stimulation painless rarely obtained with laser or electrical stimuli. Therefore, the pneumatic stimulation is a means to study allodynic evoked potentials unknown to date.